Aligning agent for liquid crystal and liquid-crystal display element

ABSTRACT

To provide a liquid crystal alignment treating agent to obtain a liquid alignment film of polyamide type which, in its applications to various display devices employing nematic liquid crystal, is excellent in voltage retention, has a reduced charge accumulation and is excellent in durability against rubbing treatment, and a liquid crystal display device employing it. 
 
A liquid crystal alignment treating agent to obtain an alignment film for nematic liquid crystal by rubbing treatment after forming a coating film, characterized in that it comprises at least one polymer selected from a polyamic acid obtained by reacting one or more tetracarboxylic dianhydrides with one or more diamines comprising at least one diamine having a structure represented by the following formula (I), and a polyimide obtained by cyclodehydration of such a polyamic acid; and a liquid crystal display device obtained by applying the liquid crystal alignment treating agent to a pair of substrates having electrodes, to form coating films, rubbing the coating film surfaces to form liquid crystal alignment films, and sandwiching nematic liquid crystal between the liquid crystal alignment films formed on the pair of substrates:  
                 
 
wherein X is a hydrogen atom or a monovalent organic group, and each of Y 1  and Y 2  is a primary amino group or a monovalent organic group having one primary amino group.

TECHNICAL FIELD

The present invention relates to a liquid crystal alignment treatingagent which presents a liquid crystal alignment film excellent inelectrical properties and reliability and also excellent in liquidcrystal alignment and durability against rubbing treatment of thecoating film, in its application to display devices employing nematicliquid crystal, and a liquid crystal display device employing such analignment film.

BACKGROUND ART

At present, as display devices employing nematic liquid crystal, atwisted nematic (TN) device having a twist angle of 90°, a supertwistednematic (STN) device having a twist angle of 180° or more, a so-calledTFT liquid crystal device using a thin film transistor, and further,various types of display devices, such as a lateral electric field typeliquid crystal display device improved in visual angle properties and avertical alignment type liquid crystal display device, are practicallyused.

To obtain a liquid crystal alignment film for such display devices, itis industrially common to employ a method wherein a solution of apolyimide precursor, a soluble polyimide or a mixture thereof, iscoated, baked and then subjected to alignment treatment by rubbing.

Properties required for the above liquid crystal alignment filmnaturally include basic properties such as transparency, heat resistanceand chemical resistance, and they also include interfacial propertieswith liquid crystal such as a good liquid crystal alignment property anda liquid crystal tilt angle having a stable and proper degree, andelectric properties such as a voltage retention characteristic and acharge accumulation characteristic when the liquid crystal displaydevice is driven.

Further, from the viewpoint of production of a liquid crystal displaydevice, storage stability of the liquid crystal alignment treatingagent, properties as a varnish such as a printing property of thetreating agent on a substrate, and properties such as durability againstscratches or scraping in the rubbing treatment of the coating film andefficient generation and release of electrostatic charges, are alsoimportant.

Of the above properties, the electrical properties such as the chargeaccumulation amount and efficient release of the accumulated charge,which are considered to be influential to a residual image phenomenon,are particularly important, and various methods have been proposed. Forexample, a liquid crystal alignment film has been proposed whereby C-Vhysteresis at the time of application of DC has been reduced byemploying, as a material for a polyimide, a diamine having a specificstructure having the molecular weight increased without having a polaratom such as an ether bond (JP-A-6-228061). Further, it has beenproposed to employ a soluble polyimide having a nitrogen atom other thanthe imide group, thereby to shorten the time until a residual image beerased (JP-A-10-104638).

However, along with the progress in high performance of a liquid crystaldisplay device, power saving of a display device and improvement of thedurability under various environments, problems have become distinctsuch that the contrast deteriorates due to a low voltage retention in ahigh temperature environment, and that persistence of vision in thedisplay is caused by accumulation of charge by continuous driving for along time. In such a case, it is difficult to solve such two problems atthe same time solely by the techniques heretofore proposed.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide a liquid crystalalignment treating agent to obtain a liquid alignment film of polyimidetype which, in its applications to various display devices employingnematic liquid crystal, is excellent in voltage retention, has a reducedcharge accumulation and is excellent in durability against rubbingtreatment, and a liquid crystal display device employing it.

The present inventors have made diligent studies on measures to overcomethe above problems, and as a result, it has been found that byincorporating a specific structure to the liquid crystal alignmenttreating agent of polyimide type, the liquid alignment property and theresistance against rubbing treatment of the coating film will beexcellent, and it is possible to improve electrical properties such asthe property of accumulation charge and the property of voltageretention.

Namely, the liquid crystal alignment treating agent of the presentinvention is a liquid crystal alignment treating agent to obtain analignment film for nematic liquid crystal by rubbing treatment afterforming a coating film, characterized in that it comprises at least onepolymer selected from a polyamic acid obtained by reacting one or moretetracarboxylic dianhydrides with one or more diamines comprising atleast one diamine having a structure represented by the followingformula (I), and a polyimide obtained by cyclodehydration of such apolyamic acid:

wherein X is a hydrogen atom or a monovalent organic group, and each ofY¹ and Y² is a primary amino group or a monovalent organic group havingone primary amino group.

BEST MODE FOR CARRYING OUT THE INVENTION

Further, the liquid crystal display device of the present invention is aliquid crystal display device obtained by applying a liquid crystalalignment treating agent comprising at least one polymer selected frompolyamic acid obtained by reacting one or more tetracarboxylicdianhydrides with one or more diamines comprising at least one diaminehaving a structure represented by the above formula (I), and a polyimideobtained by cyclohydration of such a polyamic acid, to a pair ofsubstrates having electrodes, to form coating films, rubbing the coatingfilm surfaces to form liquid crystal alignment films, and sandwichingnematic liquid crystal between the liquid crystal alignment films formedon the pair of substrates.

BEST MODE FOR CARRYING OUT THE INVENTION

Now, the present invention will be described in detail.

The liquid crystal alignment treating agent of the present invention isa liquid crystal alignment treating agent which comprises at least onepolymer (which will be generally referred to as the specific polymer)selected from a polyamic acid obtained by reacting one or moretetracarboxylic dianhydrides with one or more diamines comprising atleast one diamine having a structure represented by the above formula(I), and a polyimide obtained by cyclodehydration of such a polyamicacid. Further, the liquid crystal alignment treating agent in thepresent invention represents a solution of the above specific polymer,to be used for forming a liquid crystal alignment film.

In the formula (I), each of Y¹ and Y² is a primary amino group or amonovalent organic group having one primary amino group. The diaminehaving a structure represented by the formula (I) is a diaminecharacterized by a structure wherein at any one position of 1- to4-positions and at any one position of 5- to 8-positions, of thecarbazole structure; primary amino groups are, respectively, bondeddirectly or via another organic group. The monovalent organic grouphaving one primary amino group is not particularly limited, but, it may,for example, be an aminoalkyl group, an aminoalkoxy group, anaminophenyl group, an aminophenoxy group, an aminobenzyl group or anaminobenzoyl group. In the formula (I), the remaining positions of from1- to 8-positions of the carbazole structure may be occupied by hydrogenatoms, or may be substituted by a substituent other than a primary aminogroup, such as an alkyl group, an alkoxy group, an aromatic group, ahalogen atom, an alkyl group substituted by halogen, an alkoxyl groupsubstituted by halogen or an aromatic group substituted by halogen.

In the formula (I), X is a hydrogen atom or a monovalent organic group.X at the N-position of the carbazole structure is basically preferably ahydrogen atom, but may be substituted by a monovalent organic group.Such a monovalent organic group may, for example, be a C₁₋₂₀ alkylgroup, a C₁₋₂₀ alkenyl group, a cycloalkyl group, a phenyl group, abiphenyl group, a terphenyl group or a group comprised of a combinationthereof. Further, at the N-position in the carbazole structure,introduction of a substituent is relatively easy, and for the purpose ofimparting a further characteristic, a specific substituent may beintroduced. For example, introduction of a substituent such as a C₆₋₂₀alkyl group, a cycloalkyl group or a fluoroalkyl group, is effective toincrease the pretilt angle of liquid crystal.

The diamine having a structure represented by the formula (I) to be usedto obtain the specific polymer of the present invention, is notparticularly limited so long as the above requirements are satisfied.However, for such a reason that the density of the carbazole structurecan be made high when formed into a polyamic acid or a polyimide, themolecular weight of the diamine having a structure represented by theformula (I) is preferably as small as possible. As a preferred specificexample, a diamine of the formula (I) wherein each of Y¹ and Y² is aprimary amino group, the remaining portions of 1- to 8-positions of thecarbazole structure are occupied by hydrogen atoms, and X is a hydrogenatom, may be mentioned. More specifically, 1,5-diaminocarbazole,1,6-diaminocarbazole, 1,7-diaminocarbazole, 1,8-diaminocarbazole,2,5-diaminocarbazole, 2,6-diaminocarbazole, 2,7-diaminocarbazole,3,5-diaminocarbazole, 3,6-diaminocarbazole or 4,5-diaminocarbazole maybe mentioned. Among these diaminocarbazoles, 3,6-diaminocarbazole ismost preferred which has a high reactivity with a tetracarboxylicdianhydride and whereby a polymer having a high molecular weight can beobtained.

The diamine to be used to obtain the specific polymer of the presentinvention, is required to contain at least one diamine having astructure represented by the formula (I). In a case where a plurality ofdiamines are used in combination, other diamines may be used incombination. Such other diamines are not particularly limited, but theirspecific examples may be an aromatic diamine such as p-phenylenediamine,m-phenylenediamine, 2,5-diaminotoluene, 2,6-diaminotoluene,4,4′-diaminobiphenyl, 3,3′-dimethyl-4,4′-diaminobiphenyl,3,3′-dimethoxy-4,4′-diaminobiphenyl, diaminodiphenylmethane,diaminodiphenyl ether, 2,2′-diaminodiphenylpropane,bis(3,5-diethyl-4-aminophenyl)methane, diaminodiphenylsulfone,diaminobenzophenone, diaminonaphthalene, 1,4-bis(4-aminophenoxy)benzene,1,4-bis(4-aminophenyl)benzene, 9,10-bis(4-aminophenyl)anthracene,1,3-bis(4-aminophenoxy)benzene, 4,4′-bis(4-aminophenoxy)diphenylsulfone,2,2-bis[4-(4-aminophenoxy)phenyl]propane,2,2-bis(4-aminophenyl)hexafluoropropane, or2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane, an alicyclic diaminesuch as bis(4-aminocyclohexyl)methane, orbis(4-amino-3-methylcyclohexyl)methane, an aliphatic diamine such as1,2-diaminoethane, 1,3-diaminopropane, 1,4-diaminobutane, or1,6-diaminohexane, and a silicon diamine such as1,3-bis(3-aminopropyl)-1,1,3,3-tetramethyldicycloxane. Further, for thepurpose of increasing the tilt angle of liquid crystal, a diamine havingan alkyl group, a fluoroalkyl group, a steroid skeleton or the like inits side chain may also be used in combination. The degree of the tiltangle of liquid crystal changes depending upon the size of the sidechain or the amount of introduction, of such a diamine having a sidechain. However, if the carbon number of the side chain of the diamine isless than 6, the effect of its introduction can not be expected. In acase where the carbon number is at least 6, and the amount of such adiamine is at least 5 mol %, the effect of its introduction is large,such being preferred.

The proportion of the diamine having a structure represented by theformula (I) based on all diamines to be used for the specific polymer,is preferably from 5 to 100 mol %, more preferably from 30 to 100 mol %.If the proportion of the diamine having a structure represented by theformula (I) is small, there may be a case where the effect to reducecharge accumulation can not adequately be obtained.

The tetracarboxylic dianhydride to be used to obtain the specificpolymer of the present invention may be one tetracarboxylic dianhydrideor a combination of a plurality of tetracarboxylic dianhydrides. Thestructure of such a tetracarboxylic dianhydride is not particularlylimited. However, it is preferred to employ at least one tetracarboxylicdianhydride represented by the following formula (II):

wherein R is a tetravalent organic group having an alicyclic structure.

By using the tetracarboxylic dianhydride represented by the formula(II), the rubbing durability and the voltage retention property of theliquid crystal alignment film obtainable from the liquid crystalalignment treating agent of the present invention, will further beimproved. Here, the proportion of the tetracarboxylic dianhydriderepresented by the formula (II) based on all tetracarboxylicdianhydrides to be used for the specific polymer, is preferably from 20to 100 mol %, more preferably from 50 to 100 mol %. If the proportion ofthe tetracarboxylic dianhydride represented by the formula (II) is lessthan 20 mol %, the effect to further improve the rubbing durability andthe voltage retention property tends to be small.

Preferred examples of the tetracarboxylic dianhydrides represented bythe formula (II) include dianhydrides of e.g.1,2,3,4-cyclobutanetetracarboxylic acid,1,3-dimethyl-1,2,3,4-tetracarboxycyclobutane, 1,2,3,4-cyclopentanetetracarboxylic acid, 1,2,4,5-cyclohexane tetracarboxylic acid,2,3,5-tricarboxycyclopentyl acetic acid,3,4-dicarboxy-1,2,3,4-tetrahydro-1-naphthalene succinic acid andbicyclo[3,3,0]octane-2,4,6,8-tetracarboxylic acid.

Further, specific examples of other tetracarboxylic dianhydrides whichmay be used for the specific polymer, include dianhydrides of aromatictetracarboxylic acids such as pyromellitic acid, 2,3,6,7-naphthalenetetracarboxylic acid, 1,2,5,6-naphthalene tetracarboxylic acid,1,4,5,8-naphthalene tetracarboxylic acid, 2,3,6,7-anthracenetetracarboxylic acid, 1,2,5,6-anthracene tetracarboxylic acid,3,3′,4,4′-biphenyl tetracarboxylic acid, 2,3,3′,4′-biphenyltetracarboxylic acid, bis(3,4-dicarboxyphenyl) ether,3,3′,4,4′-benzophenone tetracarboxylic acid,bis(3,4-dicarboxyphenyl)sulfone, bis(3,4-dicarboxyphenyl)methane,2,2-bis(3,4-dicarboxyphenyl)propane,1,1,1,3,3,3-hexafluoro-2,2-bis(3,4-dicarboxyphenyl)propane,bis(3,4-dicarboxyphenyl)dimethylsilane,bis(3,4-dicarboxyphenyl)diphenylsilane, 2,3,4,5-pyridine tetracarboxylicacid and 2,6-bis(3,4-dicarboxyphenyl)pyridine, and dianhydrides ofaliphatic tetracarboxylic acids, such as 1,2,3,4-butane tetracarboxylicacid.

The method for reacting the tetracarboxylic dianhydride with the diamineto obtain the specific polymer of the present invention, is notparticularly limited. However, a method of reacting the tetracarboxylicdianhydride with the diamine in an organic solvent to obtain a polyamicacid, is simple and preferred.

The method of reacting the tetracarboxylic dianhydride with the diaminein an organic solvent, may, for example, be a method wherein a solutionhaving the diamine dispersed or dissolved in an organic solvent, isstirred, and the tetracarboxylic dianhydride is added thereto as it isor as dispersed or dissolved in an organic solvent, a method whereininversely, the diamine is added to a solution having the tetracarboxylicdianhydride dispersed or dissolved in an organic solvent, or a methodwherein the tetracarboxylic dianhydride and the diamine are alternatelyadded. In the present invention, any one of these methods may beemployed. Further, when a plurality of compounds are to be used as thetetracarboxylic dianhydrides or as the diamines, they may be reacted ina state where they are preliminarily mixed, or they may individuallysequentially be reacted.

In the case where the tetracarboxylic dianhydride is reacted with thediamine in an organic solvent, the reaction temperature is usually from0 to 150° C., preferably from 5 to 100° C. Further, the reaction can becarried out at an optional concentration, but if the concentration istoo low, it tends to be difficult to obtain a polymer having a highmolecular weight, and if the concentration is too high, the viscosity ofthe reaction solution is likely to be too high to carry out uniformstirring. Therefore, it is preferably from 1 to 50 wt %, more preferablyfrom 5 to 30 wt %. The reaction may be carried out at a highconcentration at the initial stage of the reaction, and then an organicsolvent may be added.

The organic solvent to be used for the above reaction is notparticularly limited so long as it is capable of dissolving the formedpolymer. However, specific examples thereof includeN,N-dimethylformamide, N,N-dimethylacetamide, N-methyl-2-pyrrolidone,N-methylcaprolactam, dimethylsulfoxide, tetramethylurea, pyridine,dimethylsulfone, hexamethylsulfoxide and γ-butyrolactone. They may beused alone or in combination as a mixture. Further, even a solvent whichis incapable of dissolving the polyamic acid, may be used as mixed withthe above solvent within such a range that the formed polyamic acid willnot precipitate. Further, moisture in the organic solvent may impair thepolymerization reaction and may cause hydrolysis of the formed polyamicacid. Accordingly, as the organic solvent, it is preferred to employ onewhich has been dehydrated and dried.

The ratio of the tetracarboxylic dianhydride to the diamine to be usedfor the reaction for the synthesis of a polyamic acid is preferably from1:0.8 to 1:1.2 by molar ratio. Like in a usual polycondensationreaction, the molecular weight of the polyamic acid becomes large, asthis molar ratio tends to be close to 1:1.

If the molecular weight of the polyamic acid is too small, the strengthof the coating film thereby obtainable may sometimes be inadequate. Onthe other hand, if the molecular weight of the polyamic acid is toolarge, the viscosity of the liquid crystal alignment treating agentthereby obtainable may sometimes be too high, whereby the operationefficiency during formation of the coating film or the uniformity of thecoating film is likely to be poor. Accordingly, the reduced viscosity ofthe polyamic acid (as measured at 30° C. in N-methyl-2-pyrrolidone (NMP)at a concentration of 0.5 dl/g) to be used for the liquid crystalalignment treating agent of the present invention, is preferably from0.1 to 2.0, more preferably from 0.2 to 1.5.

The specific polymer to be incorporated in the liquid crystal alignmenttreating agent of the present invention may be the polyamic acidobtained as described above, but it may be a polyimide having such apolyamic acid cyclodehydrated by heating or by means of a catalyst.However, depending upon the structure of the polyamic acid, there may bea case wherein it will be insolubilized by the imidation reaction,whereby it tends to be difficult to be used for a liquid crystalalignment treating agent. In such a case, not all of amic acid groups inthe polyamic acid may be imidated, and imidation may be carried outwithin a range where a proper solubility can be maintained.

The imidation reaction for cyclodehydration of the polyamic acid isusually carried out by thermal imidation wherein a solution of thepolyamic acid is heated as it is, or by chemical imidation wherein acatalyst is added to a solution of the polyamic acid. The chemicalimidation where the imidation reaction proceeds at a relatively lowtemperature, is preferred, since decrease in the molecular weight of theobtainable polyimide is thereby less likely to occur.

Such chemical imidation can be carried out by reacting a polyamic acidin an organic solvent in the presence of a basic catalyst and an acidanhydride at a reaction temperature of from −20 to 250° C., preferablyfrom 0 to 180° C., for a reaction time of from 1 to 100 hours. Theamount of the basic catalyst is usually from 0.5 to 30 times, preferablyfrom 2 to 20 times, by mol to amic acid groups, and the amount of theacid anhydride is usually from 1 to 50 times, preferably from 3 to 30times, by mol to amic acid groups. If the amount of the basic catalystor the acid anhydride is small, the reaction may not adequately proceed,and if it is too much, it tends to be difficult to remove it completelyafter completion of the reaction. As the basic catalyst to be used here,pyridine, triethylamine, trimethylamine, tributylamine or trioctylamine,may, for example, be mentioned. Among them, pyridine is preferred, sinceit has a proper basicity to let the reaction proceed. As the acidanhydride, acetic anhydride, trimellitic anhydride or pyromelliticanhydride, may, for example, be mentioned. Among them, it is preferredto employ acetic anhydride, whereby purification after completion of thereaction will be easy. As the organic solvent, the above-describedsolvent to be used for the preparation of a polyamic acid, may be used.The imidation degree by chemical imidation can be controlled byadjusting the amount of the catalyst, the reaction temperature or thereaction time.

For the liquid crystal alignment treating agent of the presentinvention, the reaction solution of the polyamic acid or the polyimide,obtained as described above, may be used as it is. However, the reactionsolution may be put into a poor solvent, so that the specific polymermay be precipitated, recovered and re-dissolved for use. Especially, inthe polyimide solution after the chemical imidation, the basic catalystor the acid anhydride remains, and it is preferred to precipitate andrecover the polyimide for use. The poor solvent to be used at that timeis not particularly limited, but, methanol, acetone, hexane, butylcellosolve, heptane, methyl ethyl ketone, methyl isobutyl ketone,ethanol, toluene or benzene may, for example, be mentioned. The polymercomponent precipitated by putting it in the poor solvent, is recoveredby filtration and then dried under atmospheric pressure or reducedpressure at room temperature or under heating to obtain a powder.Further, an operation of re-dissolving the precipitated and recoveredpolymer in an organic solvent, followed by reprecipitation for recovery,may be repeated for 2 to 10 times, whereby impurities in the polymer canbe minimized. In such a case, it is preferred to employ at least threetypes of poor solvents such as alcohols, ketones or hydrocarbons, as thepoor solvents, whereby the purification efficiency will be furtherincreased.

The solvent to re-dissolve the recovered specific polymer is notparticularly limited so long as it is one wherein the specific polymeris soluble. However, its specific examples includeN,N-dimethylformamide, N,N-dimethylacetamide, N-methyl-2-pyrrolidone,N-methylcaprolactam, 2-pyrrolidone, N-ethylpyrrolidone,N-vinylpyrrolidone, dimethylsulfoxide, tetramethylurea, pyridine,dimethylsulfone, hexamethylsulfoxide and γ-butyrolactone. They may beused alone or in combination as a mixture of two or more of them.

The liquid crystal alignment treating agent of the present invention isone obtained by adjusting the concentration of the solution of thespecific polymer obtained as described above. The solid contentconcentration of the liquid crystal alignment treating agent of thepresent invention may suitably be changed depending upon thepredetermined thickness of the liquid crystal alignment film to beformed, and it is preferably from 1 to 10 wt %. If it is less than 1 wt%, it tends to be difficult to form a coating film which is uniform andfree from defects, and if it exceeds 10 wt %, the storage stability ofthe solution may sometimes be poor.

The solvent to be used for adjusting the concentration may be theabove-mentioned solvent for re-dissolving the specific polymer. Further,even a solvent which is incapable of dissolving the specific polymer byitself, may be used as mixed within a range not to precipitate thepolymer component. Especially, it is known that the uniformity of thecoating film at the time of coating will be improved by incorporating ina proper degree a solvent having a low surface tension such as ethylcellosolve, butyl cellosolve, ethylcarbitol, butylcarbitol,ethylcarbitol acetate, ethylene glycol, 1-methoxy-2-propanol,1-ethoxy-2-propanol, 1-butoxy-2-propanol, 1-phenoxy-2-propanol, butyleneglycol monoacetate, propylene glycol diacetate, propyleneglycol-1-monomethyl ether-2-acetate, propylene glycol-1-monoethylether-2-acetate, dipropylene glycol, 2-(2-ethoxypropoxy)propanol, methyllactate, ethyl lactate, n-propyl lactate, n-butyl lactate or isoamyllactate. Also in the liquid crystal alignment treating agent of thepresent invention, this is suitably employed in a case where it isdifficult to form a uniform coating film with the single solventcomposition.

Further, to the liquid crystal alignment treating agent of the presentinvention, an additive such as a silane coupling agent may be added inorder to improve the adhesion of the coating film to the substrate.Further, two or more specific polymers may be mixed, or other polymercomponents may be added.

The liquid crystal alignment treating agent of the present inventionobtained as described above may be filtered, then applied to asubstrate, dried and baked to form a coating film, and this coating filmsurface is subjected to rubbing treatment, whereupon it is used as aliquid crystal alignment film for nematic liquid crystal.

The substrate to be used here is not particularly limited so long as itis a substrate having high transparency. A glass substrate or a plasticsubstrate such as an acrylic substrate or a polycarbonate substrate,may, for example, be used. With a view to simplifying the process, it ispreferred to employ a substrate having ITO electrodes, etc. formed todrive liquid crystal. Further, in the case of a reflection type liquidcrystal display device, an opaque substance such as a silicon wafer maybe used for a substrate on one side only, and in such a case, for theelectrodes, a material which reflects light, such as aluminum may alsobe used.

As the method for coating the liquid crystal alignment treating agent, aspin coating method, a printing method or an ink jet method may, forexample, be mentioned. From the viewpoint of the productivity, atransfer printing method is widely used industrially, and it can alsosuitably be used with the liquid crystal alignment treating agent of thepresent invention.

The drying step after coating the liquid crystal alignment treatingagent is not necessarily required. However, it is preferred toincorporate a drying step in a case where the time until baking afterthe coating is not constant for every substrate or in a case wherebaking is not immediately carried out after the coating. This drying issufficient if the solvent is evaporated to such an extent that the shapeof the coating film undergoes no change by e.g. transportation of thesubstrate, and the drying means is not particularly limited.Specifically, a method of drying on a hot plate of from 50 to 150° C.,preferably from 80 to 120° C., for from 0.5 to 30 minutes, preferablyfrom 1 to 5 minutes, may be adopted.

Baking of the liquid crystal alignment treating agent may be carried outat an optional temperature of from 100 to 350° C., preferably from 150°C. to 300° C., more preferably from 200° C. to 250° C. In a case wherethe liquid crystal alignment treating agent contains a polyamic acid,the conversion of the polyamic acid to a polyimide changes dependingupon this baking temperature. However, in the present invention, theliquid crystal alignment treating agent may not necessarily be imidated100%. However, it is preferred to carry out the baking at a temperaturehigher by at least 10° C. than the temperature for heat treatment fore.g. curing the sealing agent, which is required in the process forpreparing a liquid crystal cell.

The thickness of the coating film after the baking is usually from 5 to300 nm, preferably from 10 to 100 nm, since if it is too thick, such isdisadvantageous from the viewpoint of the power consumption of theliquid crystal display device, and if it is too thin, the reliability ofthe liquid crystal display device may sometimes decrease.

The coating film of the liquid crystal alignment treating agent of thepresent invention may be made to be a liquid crystal alignment film fornematic liquid crystal by rubbing treatment i.e. an operation of rubbingthe coating film surface in a predetermined direction with acommercially available rubbing cloth. The material for the rubbing clothmay, for example, be nylon, rayon or cotton, but it is not particularlylimited thereto.

The liquid crystal display device of the present invention is oneobtained by preparing a liquid crystal cell, as a liquid crystal displaydevice, by a known method by using a nematic liquid crystal, afterobtaining the substrate having the liquid crystal alignment film fromthe liquid crystal alignment treating agent of the present invention bythe above-described method. As an example for preparation of the liquidcrystal cell, a method is common wherein a pair of substrates havingliquid crystal alignment films formed thereon are disposed with a spacerof from 1 to 30 μm, preferably from 2 to 10 μm, interposed, so that therubbing directions will take an optional angle of from 0 to 270°, theperiphery is fixed by a sealing agent, then liquid crystal is injected,followed by sealing. The method for injecting liquid crystal is notparticularly limited, and a vacuum method wherein liquid crystal isinjected after evacuating the interior of the liquid crystal cellprepared, or a dropping method wherein after dropping liquid crystal,sealing is carried out, may, for example, be mentioned.

The liquid crystal display device thus prepared by using the liquidcrystal alignment treating agent of the present invention, has excellentelectrical properties and thus can be made to be a liquid crystaldisplay device which is less susceptible to decrease of the contrast orpersistence of vision. For example, it is useful for display devices ofvarious systems employing nematic liquid crystal, such as TN devices,STN devices, TFT liquid crystal devices, as well as lateral electricfield type liquid crystal display devices and vertical alignment typeliquid crystal devices.

Now, the present invention will be described in further detail withreference to Examples. However, it should be understood that the presentinvention is by no means restricted thereto. In the following, thereduced viscosities of polyamic acids are values measured at 30° C. inNMP at a concentration of 0.5 dl/g.

EXAMPLES Preparation Example 1

In a nitrogen stream, in a 100 mL four-necked flask, 0.99 g (0.005 mol)of 3,6-diaminocarbazole (hereinafter referred to simply as DCA) wasdissolved in 10 g of N-methyl-2-pyrrolidone (hereinafter referred tosimply as NMP), and then a solution having 0.94 g (0.0048 mol) of1,2,3,4-cyclobutane tetracarboxylic dianhydride (hereinafter referred tosimply as CBDA) suspended in 7.35 g of NMP, was put, followed bypolymerization for 20 hours to obtain a solution of polyamic acid (A-1).The polymerization reaction proceeded easily and uniformly, and thereduced viscosity of this polyamic acid was 1.50 dl/g.

Preparation Example 2

In a nitrogen stream, in a 100 mL four-necked flask, 1.06 g (0.0054 mol)of DCA and 0.23 g (0.0006 mol) of 1,3-diamino-4-octadecyloxybenzene weredissolved in 10 g of NMP, and then, a solution having 1.15 g (0.0059mol) of CBDA suspended in 7.35 g of NMP, was put, followed bypolymerization for 20 hours to obtain a solution of polyamic acid (A-2).The polymerization reaction proceeded easily and uniformly, and thereduced viscosity of this polyamic acid was 1.20 dl/g.

Preparation Example 3

In a nitrogen stream, in a 100 mL four-necked flask, 0.95 g (0.0048 mol)of CDA and 1.43 g (0.0072 mol) of 4,4′-diaminodiphenylmethane(hereinafter referred to simply as DDM) were dissolved in 15 g of NMP,and then a solution having 2.34 g (0.0119 mol) of CBDA suspended in11.72 g of NMP, was put, followed by polymerization for 20 hours toobtain a solution of polyamic acid (A-3). The polymerization reactionproceeded easily and uniformly, and the reduced viscosity of thispolyamic acid was 0.90 dl/g.

Preparation Example 4

In a nitrogen stream, 10.38 g (0.096 mol) of p-phenylenediamine and19.61 g (0.1 mol) of CBDA were reacted in 341.2 g of NMP at roomtemperature for 5 hours to obtain a solution of polyamic acid (B-1). Thepolymerization reaction proceeded easily and uniformly, and the reducedviscosity of this polyamic acid was 1.20 dl/g.

Preparation Example 5

In a nitrogen stream, 10.91 g (0.1 mol) of 2,6-diaminopyridine and 19.52g (0.0995 mol) of CBDA were reacted in 121.7 g of NMP at roomtemperature for 20 hours to obtain a solution of polyamic acid (B-2).The reduced viscosity of the obtained polyamic acid was 0.55 dl/g.

Preparation Example 6

In a nitrogen stream, 13.88 g (0.07 mol) of DDM and 13.66 g (0.0696 mol)of CBDA were reacted in 156.04 g of NMP at room temperature for 20 hoursto obtain a solution of polyamic acid (B-3). The polymerization reactionproceeded easily and uniformly, and the reduced viscosity of thispolyamic acid was 1.10 dl/g.

Example 1

The solution of polyamic acid (A-1) obtained in Preparation Example 1,was diluted with NMP to obtain a liquid crystal alignment treating agentof the present invention having a resin concentration of 4 wt %.

Preparation of Liquid Crystal Cell

The above liquid crystal alignment treating agent was applied by spincoating on the ITO surface of a glass substrate provided with ITOelectrodes, dried at 80° C. for 5 minutes and then baked at 250° C. for60 minutes to obtain a coating film having a thickness of 100 nm. Thecoating film surface was subjected to rubbing treatment is by a rubbingapparatus having a roll diameter of 120 mm with a rayon cloth under suchconditions that the rotational speed was 500 rpm, the mobile speed was20 mm/sec and the pressing amount of 0.6 mm, to obtain a liquid crystalalignment film. Two sheets of such a substrate provided with the liquidcrystal alignment film, were prepared. A spacer having a diameter of 6μm was applied to the liquid crystal alignment film surface of onesubstrate. Then, the two substrates were combined so that the rubbingdirections crossed each other, and the periphery was sealed except forthe injection inlet for liquid crystal, to obtain a vacant cell having acell gap of 6 μm. To this cell, nematic liquid crystal (MLC-2003C,manufactured by Merck), was injected under vacuum at room temperature,and the injection inlet was sealed to obtain a twist nematic liquidcrystal cell.

Evaluation of Rubbing Durability and Liquid Crystal Orientation Property

Evaluation of the rubbing durability was carried out by observing thesurface of the liquid crystal alignment film after the rubbingtreatment, by a polarizing microscope, whereby one having scraping ofthe film observed, was regarded as “no good”. Further, evaluation of theliquid crystal alignment property was carried out by observing theliquid crystal cell immediately after the preparation under crossedNicols, whereby one having disoriented alignment observed was regardedas “no good”.

Evaluation of Voltage Retention and Charge Accumulation

Evaluation of the voltage retention was carried out by applying avoltage of 4 V for 60 μsec to a liquid crystal cell set at a temperatureof 23° C. or 90° C. and measuring the voltage upon expiration of 16.67msec, whereby the percentage of the voltage maintained was calculated asthe voltage retention. Further, evaluation of the charge accumulationwas carried out by applying rectangular waves of 30 Hz/±3 V havingdirect current 3 V superimposed, at 23° C. for 60 minutes and measuringthe accumulated voltage remaining in the liquid crystal cell immediatelyafter switching off the direct current voltage of 3 V, by an opticalflicker elimination method.

As a result of the above evaluation, no scraping was observed on thefilm after rubbing, and no disoriented alignment was observed in theliquid crystal cell. Further, this liquid crystal cell had a voltageretention of 99% at 23° C. and a voltage retention of 96% at 90° C., andthe accumulated voltage was 0 V. This result is also shown in Table 1given hereinafter.

Example 2

A liquid crystal cell was prepared and evaluated in the same manner asin Example 1 except that in Example 1, the baking of the coating filmwas carried out at 220° C. for 30 minutes. The results of evaluation areshown in Table 1 given hereinafter.

Example 3

The solution of polyamic acid (A-2) obtained in Preparation Example 2was diluted with NMP to obtain a liquid crystal alignment treating agentof the present invention having a resin concentration of 4 wt %. Usingthis liquid crystal alignment treating agent, a liquid crystal cell wasprepared and evaluated in the same manner as in Example 1. The resultsof evaluation are shown in Table 1 given hereinafter.

Example 4

The solution of polyamic acid (A-3) obtained in Preparation Example 3was diluted with NMP to obtain a liquid crystal alignment treating agentof the present invention having a resin concentration of 4 wt %. Usingthis liquid crystal alignment treating agent, a liquid crystal cell wasprepared and evaluated in the same manner as in Example 1. The resultsof evaluation are shown in Table 1 given hereinafter.

Comparative Example 1

The solution of polyamic acid (B-1) obtained in Preparation Example 4was diluted with NMP to obtain a liquid crystal alignment treating agenthaving a resin concentration of 4 wt %. Using this liquid crystalalignment treating agent, a liquid crystal cell was prepared andevaluated in the same manner as in Example 1. The results of evaluationare shown in Table 1 given hereinafter.

Comparative Example 2

The solution of polyamic acid (B-2) obtained in Preparation Example 5was diluted with NMP to obtain a liquid crystal alignment treating agenthaving a resin concentration of 4 wt %. Using this liquid crystalalignment treating agent, a liquid crystal cell was prepared andevaluated in the same manner as in Example 1. The results of evaluationare shown in Table 1 given hereinafter.

Comparative Example 3

The solution of polyamic acid (B-3) obtained in Preparation Example 6was diluted with NMP to obtain a liquid crystal alignment treating agenthaving a resin concentration of 4 wt %. Using this liquid crystalalignment treating agent, a liquid crystal cell was prepared andevaluated in the same manner as in Example 1. The results of evaluationare shown in Table 1 given hereinafter. TABLE 1 Voltage Accumu- Liquidretention lated crystal (%) voltage orientation Rubbing Polymer 23° C.90° C. (V) property durability Ex. 1 A-1 99 96 0 Good Good 2 A-1 99 86 0Good Good 3 A-2 99 97 0 Good Good 4 A-3 99 94 0.2 Good Good Comp. Ex. 1B-1 99 77 0.8 Good No good 2 B-2 99 93 0.3 No good No good 3 B-3 99 881.5 Good Good

INDUSTRIAL APPLICABILITY

By the liquid alignment treating agent of the present invention, it ispossible to obtain a liquid alignment film which, in its applications tovarious display devices employing nematic liquid crystal, is excellentin voltage retention, has a reduced charge accumulation and is excellentin durability against rubbing treatment. A liquid crystal display deviceemploying the liquid crystal alignment film of the present invention isless susceptible to deterioration of the contrast or persistence ofvision and thus is useful as a display device of a various systememploying nematic liquid crystal, such as a TN device, a STN device, aTFT liquid crystal device, or a lateral electric field type liquidcrystal display device or a vertical alignment type liquid crystaldisplay device.

1. A liquid crystal alignment treating agent to obtain an alignment filmfor nematic liquid crystal by rubbing treatment after forming a coatingfilm, characterized in that it comprises at least one polymer selectedfrom a polyamic acid obtained by reacting one or more tetracarboxylicdianhydrides with one or more diamines comprising at least one diaminehaving a structure represented by the following formula (I), and apolyimide obtained by cyclodehydration of such a polyamic acid:

wherein X is a hydrogen atom or a monovalent organic group, and each ofY¹ and Y² is a primary amino group or a monovalent organic group havingone primary amino group.
 2. The liquid crystal alignment treating agentaccording to claim 1, wherein the diamine having a structure representedby the formula (I) is 3,6-diaminocarbazole.
 3. The liquid crystalalignment treating agent according to claim 1, wherein said one or moretetracarboxylic dianhydrides are one or more tetracarboxylicdianhydrides comprising at least one tetracarboxylic dianhydriderepresented by the following formula (II):

wherein R is a tetravalent organic group having an alicyclic structure.4. A liquid crystal display device obtained by applying the liquidcrystal alignment treating agent as defined in any one of claims 1 to 4to a pair of substrates having electrodes, to form coating films,rubbing the coating film surfaces to form liquid crystal alignmentfilms, and sandwiching nematic liquid crystal between the liquid crystalalignment films formed on the pair of substrates.